The present invention refers to a device of a foil system for hydrofoil craft, where the foil system comprises a number of adjustable control fins/surfaces, struts between the foil system and the hull of the craft, and means to regulate the control fins/surfaces, and at speed, the foil system is arranged to be completely submerged and lifts and carries the total weight of the craft.
One of the problems associated with obtaining high speeds in conventional marine craft is the wave pattern by which these are surrounded. With increasing speed the resistance will increase rapidly even if the craft has a favourable shape. To reduce this problem it is known technology to employ surfaces with dynamic lift. In this way the craft is lifted to a greater or lesser degree out of the water and the wave generated resistance is reduced.
In the case that these surfaces consist of the craft""s bottom (planing craft) the surfaces must necessarily be so large that the shearing forces (the viscous forces) become large and the resistance will consequently be large at high speeds. The most effective surfaces to create lift with least possible resistance are wing profiles (foils). These wing profiles can be either completely submerged or piercing the water surface. Such marine craft where the foil system lifts and carries the total weight of the craft at speed, are known as hydrofoil craft, and reference is made to U.S. Pat. Nos. 2,749,871, 3,092,062 and 3,137,260.
The Swedish patent No. SE-331.802 discloses a foil structure which is intended for piercing the water surface. Said foil structure consists of a foremost foil member cooperating with a central main foil member which carries the main of the craft""s weight. The foremost foil member carries the remainder weight of the craft. Thus the structure always operates with two separate foil members. The foremost foil comprises two central sections of the body making an angle of approx. 35xc2x0 with the horizontal, and two perphery sections each with an angle of 15xc2x0. The outer inclined side sections of the foil body are arranged to pierce the water surface. Thus some of said foil body is always above the water level at speed, thus exhibiting an upper dry surface. The foil is therefore partly planing on/along the surface of the sea. The foremost foil member is necessary to stabilize the craft in pitch at high speed, when riding on the main foil member which is positioned amidships. The foil structure does not include contol surfaces to control the vessel, and therefore it is self-adjusting.
By providing the dynamic lift from either the bottom section of a craft or from a foil system piercing the water surface, the variation in vertical forces will become large. This leads to high accelerations on the craft and unpleasantness for those aboard (Ref. ISO 2631). This may be avoided to a greater or lesser degree by moving the lifting surfaces completely below the water surface, where the conditions are calmer. This has proven to be the superior method in many ways and has been used on several hydrofoil craft, but has also led to a number of disadvantages.
The lift from a wing depends on the geometric profile of the wing, the mass density of the water, the projected area of the wing in the vertical sense and the relative velocity between the wing and the surrounding water. Since the lift must have the same magnitude as the total weight of the craft, and since both weight and velocity can vary it must be possible to vary the geometry of the wing, alternatively to vary the area of the wing as is done for a hydrofoil craft with surface piercing wings. The variation in the geometry of the wing is commonly effected by mounting several flaps on the following edge of the wing or wings.
The present invention relates to a foil system including only one foil element with accompanying control surfaces which by their arrangement can control a craft having its hull at some distance above the water both in pitch and roll.
For existing or disclosed hydrofoil craft the motions of the craft can be monitored or measured, so that these measurements can be used in a control system for continuous adjustment of the flap angle on the different wings and hence assure that the lifting force at any time is adapted to the craft""s weight, the location of the centre of gravity and existing mass forces. This will ensure a very pleasant voyage even in rough seas, and has been employed on several types of hydrofoil craft. All known hydrofoil craft utilise either several wings or a combination of dynamic lift from one or several wings combined with buoyancy from one or more submerged hulls in order to maintain longitudinal and/or transverse stability.
By using dynamic lift from several wings or by combining lift from wings and hulls, however, other difficulties and disadvantages are introduced. Of particular importance in the context of the present invention, the following five issues can be mentioned:
i) During the take-off stage the resistance of the wings will be disproportionally larger than at full speed. This is associated with the high lift coefficient which is necessary at low speeds, and which partly is caused by the low speed, but which also can be ascribed to the low aspect ratio usually found on such wings, and which leads to an unusually high induced drag. At the same time the resistance of the hull will be high, which is associated with the desire to obtain dynamic lift from the hull itself in order to reduce the resistance at high speed, but gives at the same time rise to a high resistance curve at low speeds. The resistance curve for a hydrofoil craft will consequently often have a pronounced peak (a hump) resulting in engine overload in this speed regime.
By an aspect ratio is meant the ratio between the span and the chord of the wing, the span being the extent of the wing in the transverse direction of the craft, and the chord is the extent of the wing in the longitudinal direction of the craft, in a typical section.
ii) Since the bottom of the hull itself must be relatively flat, great clearance between the sea surface and the bottom will be necessary in order to avoid high slamming loads in waves. Such slamming loads will result in damage to the flat bottom, or if they are not that fierce, to discomfort in the behaviour of the craft and anxiety among the passengers.
iii) this large clearance will also lead to problems with the propulsors (the drive means), and if they consist of some sort of propellers they must be fixed to long struts, and the powering shafts become long and the propellers attain a sharp inclination relative to the water stream. If they consist of water jets the large distance between the inletsxe2x80x94which must be located at some distance below the water surface, and the pumpsxe2x80x94which have to be located inside the hull, will lead to large losses in the inlet, both due to the length of the intake and the abrupt velocity changes necessary to bring the water up to the hull where the pumps are located.
iv) the large clearance desirable to maintain the distance between the wings and the hull involves an unallowably large draught at zero speed. This entails that the wings must be supported by struts which can be lifted/folded in one manner or another. Such compliant structures involve introduction of mechanisms both to ensure the folding, but also mechanisms transferring forces and moments down to the wings. This system will hence be more complicated and demand more maintenance than a fixed system.
v) Since the hull itself must be kept at adequate distance from the undisturbed water surface, this means that a possible failure in the control system or a breakage of one or several of the struts or wings, brought about by e.g. collision with a log, will lead to a fall against the water surface before the buoyancy of the hull again balances the weight of the craft. The potential energy such a craft contains due to the height above the water surface, means that such a fall can be very dramatic with a violent and uncontrolled retardation of the speed. For passengers not being secured such an event can be very dramatic.
It is possible to improve considerably on the five crucial points mentioned above by using a different configuration of the hull and the wings than that which has been used thus far. All the points which have been highlighted are connected to the hull shape and consequences of this, and hence it is not the principle of providing a dynamic lift by submerged and controlled wings which is the main idea with this new construction, but it is the shape of that part of the wing which at speed lifts the craft above the water surface, and the cooperation between the design of the wing and its control surfaces, which are considered to be a new invention.
The foil system according to the invention is characterised in that the foil system is formed by one single horizontal foil body comprising generally mirror symmetric foil arms which extend aft to each side in equal angles, the forward apex of the foil arms lying in the centre line of the craft.
According to a preferred embodiment the ratio between the longitudinal extent of the foil body LF and the length of the craft LS is in the order of ⅓ to ⅘, and preferably in the order of xc2xd to xc2xe.
Further preferred embodiments of the foil system according to the invention appear from the further dependent patent claims.
According to the invention it is hence suggested to supply all the lift by means of one single wing which, in order to provide control in pitch, is fitted with control fins both in the forward apex (that is to say in the aft part of the forward apex) as well as on the two aft angled legs and which further is fitted with control fins symmetrically located on the two aft angle legs in order to provide control in roll. Additionally this wing is utilised on an unusual broad craft, which primarily, but not necessarily, must consist of several connected hulls, where use of several hulls would make it advantageous to make the outer hulls considerably more slender than the centre hull. In this way it is possible to obtain a considerably improved aspect ratio for the lifting wing, thus leading to less resistance than on known hydrofoil craft. By using as few control fins as three, which nonetheless will provide control in both pitch and roll, a more precise adjustment of the craft""s motion will be obtained than if more control fins had been used.